The present invention relates to integrated circuits, and more particularly to regulating the amplitude of an oscillating signal.
Crystal oscillators are widely used in electronic circuits and use stable electromechanical resonance characteristics of a piezoelectric crystal, such as quartz, to generate a clock signal. When a quartz crystal is cut along certain crystal axes to form parallel-plate electrodes, it can be made to resonate when excited. The resonant frequency depends on the material and dimensional properties of the crystal, such as the size, the orientation of the crystal faces, and the mounting of the crystal on its mechanical supports. Because of their insensitivity to time and temperature variations, crystal oscillators are widely used in applications requiring clock signals.
FIG. 1 is a transistor schematic diagram of an oscillator 10 as known in the prior art. Oscillator 10 is shown as including a crystal 20 disposed between input and output terminals of a CMOS inverter formed by PMOS transistor 12 and NMOS transistor 14. The oscillation signal generated between nodes X1 an X2 has an amplitude that varies between supply voltages Vdd and ground. Oscillator 10 is thus inappropriate for use in applications requiring relatively high accuracy oscillation frequency driven by small time-varying signals.